Problem Description
Implement Merge Sort.
Solution
We will make use of a problem we solved earlier: Sort a sub-array given s,m,e. If we carefully think about any given array, we can divide it into two parts -> Sort them separately -> Then merge both sorted array. For the first half of the array, we can again sort it in the same way. And keep doing it recursively until we have just a single element (which is going to be our base case). Since we have a merge function which takes s, m and e we can use this to merge two sorted array. The important part of the merge function is our assumption. The assumption we take for this recursive function is: Given A[], sort it from start(s) to end(e)
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package com.gauk;
import java.util.Arrays;
public class Sorting {
public static void main(String[] args) {
int[] arr = new int[]{4,8,-1,2,6,3,4,7,13,0};
mergeSort(arr, 0, arr.length-1);
System.out.println(Arrays.toString(arr));
}
public static void mergeSort(int[] arr, int s, int e){
if(s==e) return;
int m = (s+e)/2;
mergeSort(arr, s, m);
mergeSort(arr, m+1, e);
merge(arr, s, m, e);
}
//Given an array and index s, m and e, Sort the sub-array from s to e. Consider [s,m] and [m+1,e] is already sorted
public static void merge(int[] arr, int s, int m, int e){
int[] temp = new int[e-s+1];
int i=s;
int j=m+1;
int idx=0;
while(i<=m && j<=e){
if(arr[i] < arr[j]){
temp[idx] = arr[i];
i++;
}else{
temp[idx] = arr[j];
j++;
}
idx++;
}
for(int x=i; x<=m; x++){
temp[idx] = arr[x];
idx++;
}
for(int x=j; x<=e; x++){
temp[idx] = arr[x];
idx++;
}
for(int w=s; w<=e; w++){
arr[w] = temp[w-s];
}
}
}